Slag is ordinarily likely to flow out into a stream of a molten steel in a final stage of discharging the molten steel from a refining furnace such as a converter to a ladle through a molten steel-discharging opening, or in a final stage of pouring the molten steel from a ladle to an intermediate vessel such as a tundish through a nozzle.
If the slag is discharged into the molten steel, alloying components such as Al, Fe-Mn and Fe-Si added thereto are taken into the slag, and production cost rises due to reduction in yields of such alloying components. Further, since the molten steel is oxidized with the slag discharged, cleanness of the steel is deteriorated, so that the quality of steel products is adversely affected. For this reason, it is an extremely important control item to suppress the outflow of the slag into the molten steel to the minimum, and various countermeasures have formerly been adapted for this purpose.
As the conventional techniques for detecting the outflow of the slag into the molten steel stream, visually judging is a main technique. As methods for detecting the slag entering the stream extracted, particularly, from the ladle to the tundish, for instance, Japanese Patent Application Laid-open No. 57-112,963 discloses a process for measuring vibrations, Japanese Patent Application Laid-open No. 53-53,521 discloses a process for measuring the impedance, Japanese Patent Application Laid-open Nos. 60-3,955 and 60-3,956 disclose a process for measuring microwaves, and Japanese Patent Application Laid open No. 61-262,454 discloses a process for measuring the internal pressure of a nozzle.
However, the above-mentioned conventional techniques have the following problems.
That is, the above visual judgment lacks accuracy, because variations occur due to individual differences among judging persons. The judgment needs longer time, and it is impossible to make any such judgment in the case where a poured molten steel as in a sealed type tundish is not observable from the outside.
The vibration-measuring process, the impedance-measuring process, and the microwave-measuring process require that a measuring sensor is caused to approach the extracted stream. Thus, problems exist with respect to maintenance or operability. Furthermore, the apparatus disadvantageously becomes great size, and costly.
In the nozzle internal pressure-measuring process, a pressure-measuring hole is liable to be closed with the molten steel or the slag on measuring a negative pressure inside a long nozzle. Thus, the pressure cannot be detected in many cases. Moreover, since the change of pressure inside the nozzle is as extremely small as about 0.02 kgf/cm.sup.2 when the poured melt stream is changed from the molten steel to the slag, it is difficult to accurately detect the change. In addition, since the pressure loss is great depending upon the shape of the pressure-measuring hole, it may become impossible to detect the pressure change due to the slag discharging. Thus, the pressure cannot accurately be detected. Furthermore, the internal pressure of the nozzle detected by this method is measured by press fitting a long nozzle to a nozzle of the ladle, purging the inside of the nozzle through blowing an inert gas upon a press-fitted portion via an inert gas-blowing pipe, and measuring the static pressure (negative pressure) inside the nozzle. Although the inclusion amount of the inert gas into the molten steel stream differs from the inclusion amount of the inert gas owing to the discharging slag (the amount of the gas sucked through the press-fitted portion), the inert gas is sucked through the press-fitted portion so that the internal pressure may be constant irrespective of the flow-down kinetic energy of the flowing material. Therefore, since the internal pressure inside the nozzle is maintained at almost the same level, this process has a shortcoming in that the discharging of the slag cannot stably or accurately be detected.